Analytical techniques are employed in many different fields for determining various properties of materials. Chromatographic analysis is one commonly used analytical technique. Automatic samplers for chromatographic analysis greatly reduce both the time and labor which are required for analysis of samples. The potential of operator exposure to dangerous or hazardous chemicals is also reduced by automatic sampler devices. However, a conventional automatic sampler container consists of a glass vial and a crimped septum cap and performs only marginally when used for analysis of volatile organic compound samples. Leakage through the septum and around the seal is common. Additionally, after the septum has been pierced by a syringe needle, leakage of volatile materials from the vial is rapid. While this is of little consequence for volatile organic compounds that are sampled and analyzed quickly, it is a serious problem for samples which are used on a long term basis, as is often the case for samples comprising retention time and retention index standards, commonly referred to as markers. While it is possible to replace the crimped septum cap after every use, this approach is inconvenient, time consuming, and costly, and increases the risks of operator exposure to and accidental spillage of the sample. Moreover, leakage around the seal may persist even if the cap is replaced after each use.
Reliable gas chromatographic identification of samples requires the calculation of retention parameters. The calculation of these retention parameters requires that the experimentally-accessible retention parameter, the retention time, t.sub.r, be corrected for the gas hold-up (sometimes referred to the dead volume) of the gas chromatographic system, usually represented as t.sub.m. Generally, the retention time of an unretained sample component is subtracted from the retention time of a component of interest. When using a thermal conductivity detector, the air or nitrogen peak is often used in this capacity; however, since air does not cause a response in a flame ionization detector, other methods of correcting for gas hold-up have been devised. A number of mathematical techniques require extensive retention time measurements to be conducted on a series of C5-C10 n-alkanes to provide an accurate extrapolation. Other methods of correcting for gas hold-up involve the use of hydrodynamic models of the gas chromatographic column. However, most of these parameters are difficult to measure.
The use of methane as a minimally retained marker is a simple way to approximate gas hold-up under certain conditions, and is especially useful when higher column temperatures (approximately 80-90.degree. C. or higher) are employed with moderately polar to polar open tubular columns. When used properly and under appropriate conditions, the methane marker technique will produce minimal departures from corrections calculated using one of the mathematical approximations.
One conventional technique for delivering methane into a liquid sample, referred to as methanization, is to bubble methane gas into the liquid sample immediately prior to injection of the sample into the chromatographing apparatus. However, this type of methanization suffers from major disadvantages. Methane markers introduced by bubbling methane into a liquid sample will last only a short time (typically 30 minutes or less), and reintroduction of methane into sample vials between chromatographic runs is inconvenient, especially if crimp closure-type automatic sample vials are used. This procedure increases the likelihood of operator exposure to samples. Additionally, bubbling methane through such samples can itself be hazardous and may be impossible to implement in explosion-proof laboratories. Further, if laboratory-utility natural gas is used as a methane source, samples are exposed to numerous non-methane constituents.
The Elahi U.S. Pat. No. 4,131,154 discloses the encapsulation of a sorbent element within a porous filter membrane, and the use of such an element to remove matter from a liquid or gaseous medium. Discrete capsules of the sorbent within an encapsulating membrane are dropped into a vial containing a liquid system and the vial is gently agitated in order to remove the desired matter. Such a system is useful for determining the type or quantities of drugs contained in a biological fluid system, such as blood or urine. However, such a system does not allow for delivery of a marker material such as methane to the fluid system, and neither does it provide for long term storage of samples.
The Graas U.S. Pat. No. 4,270,921 discloses a combination of a microcolumn packed with absorbent material and a centrifuge tube. Centrifugation of the assembly of the microcolumn and the centrifuge tube causes the passage of a predetermined volume of eluant through the microcolumn. While such a system allows for removal of a substance from a fluid, it does not allow for the delivery of a marker such as methane. Such a device still would require transfer of the liquid sample from the centrifuge tube to the sample vial, thereby exposing the operator to the sample, and would solve none of the problems described above which are associated with long term storage of samples within the automatic sampler vials.
The Ullman U.S. Pat. No. 4,624,929 discloses a device for collecting a liquid sample and diluting that sample. The device comprises a housing adapted for mating with a container to form a chamber, and a bibulous pad attached to the housing for collecting a predetermined amount of liquid sample. The device creates a pressure differential sufficient to move a predetermined volume of liquid through the bibulous pad into the housing. Such a device is not capable of delivering a methane marker into a liquid sample system. Additionally, such a device is not suitable as a long term storage container of volatile organic compounds, for such volatile compounds would be lost through the bibulous pad.
The Peterson et. al. U.S. Pat. No. 4,451,374 discloses a method of adding reagent to liquid chromatographic effluents. Hollow fibers are immersed within mobile reagent which permeates through the walls of the fibers and diffuses into the column effluent. While this device is capable of delivering a liquid phase reagent into column effluent, it would not be suitable for delivery of a gaseous marker compound. In addition, such a method requires a large volume of the agent to be introduced, provides no method of sample storage, and exposes the operator to a large volume of reagent.
The prior art offers no solution for the problem of long term storage of volatile organic compounds in commercially available automatic sampler containers, or the problem of long term delivery of marker compounds. Thus, the need remains for devices which will not only allow for long term sample storage and marker compound delivery, but which will also allow for use of small quantities of compounds and which will decrease compound handling and likelihood of operator exposures.